The invention relates to an epoxy foam and a process for making the same. More particularly, this invention relates to a tailorable epoxy foam encapsulant formed using at least two resins and at least two curing agents.
In the fabrication of electronic devices and circuits, the electronic devices are protected from the adverse effects of the environment by filling the empty spaces between components on the device with a potting material that encapsulates these components. If the electronic device is used outdoors, such as in automotive, aircraft, marine, or building applications, the potting or encapsulant material must be stable enough to provide protection over extreme outdoor temperatures and humidity conditions, as well as mechanical stresses. Additionally, the encapsulant material can serve to isolate the protected devices from human intrusion. In some applications, future access to the device is desired and so the encapsulant must be capable of isolating the device for a period of time but must have mechanical and structural characteristics to allow subsequent removal of the encapsulant. Consequently, such encapsulants must possess suitable thermal, mechanical and electrical properties.
Epoxy compounds have been widely used as adhesives, encapsulants and coatings for a variety of applications. Epoxy compounds typically have good insulating properties and excellent adhesion, and are easy to process. However, flexibilized epoxies usually have poor thermal stability, hydrolytic stability and mechanical properties; and consequently, have not been used for elastomeric applications. Rather, polyurethane or silicone potting compositions have been used for these applications. However, polyurethanes are more difficult to process due to their tendency to foam when in contact with moisture-containing surfaces. Silicones have the disadvantage of being poor adhesives and are also mechanically weak and costly.
Epoxy foam encapsulants have a general composition that includes a resin, a curing agent, a surfactant, a blowing agent and a filler or nucleating agent. The resin, curing agent and surfactant, as well as the curing conditions, are chosen to yield a specific foam encapsulant structure. The mixture is cast into a mold and heated to form and cure the encapsulant.
Gormley et al. (U.S. Pat. No. 4,090,986, issued on May 23, 1978) describe an epoxy foam composition which uses a delta-1-tetrahydrophthalic anhydride epoxy resin with an amine curing agent to form a thermoset epoxy foam. Hermansen et al. (U.S. Pat. No. 5,457,165, issued on Oct. 10, 1995) note that there are various resins which may be used in forming an encapsulant; diglycidyl ethers are commonly used. Hermansen et al. also describe an encapsulant which uses two resins to tailor the property of the formed encapsulant to provide both dry heat and humid heat stability. The invention according to Hermansen et al. differs from other formed encapsulants in that the invention utilizes more than one resin to tailor specific properties of the encapsulant to achieve desired properties. Russick and Rand (Sandia National Laboratories Technical Report SAND98-2538, Albuquerque, N. Mex., December, 1998, incorporated by reference herein) describes a new epoxy foam encapsulant that utilizes multiple resins and curing agents to control the reactivity, exotherm, and pore size and therefore the mechanical, thermal, and electrical properties of the formed epoxy foam encapsulant. Such control allows for epoxy foams of larger volumes without excessive reactivity and exotherm which could lead to lowered pot life, premature curing prior to filling void volume, or burn out of the foam.
Useful would be a foam encapsulant without carcinogenic constituents that has tailorable mechanical, thermal, and electrical properties that allow encapsulation of a component such as electromechanical components or instrumentation that sufficiently isolates the material from the outside environment with adequate thermal/mechanical properties to allow removal so that the encapsulated component can be accessed.